This invention relates to the driving of lasers at various currents to select optical power output and at high speeds characteristic of high-resolution laser printing.
Standard lasers have a diode mounted behind the light source, known as a PIN diode, which receives a small, fixed proportion of the light energy of the laser, and it is widely practiced to calibrate lasers by adjusting current to the laser in response to the output of the PIN diode. Such systems typically are for duty cycles (on to off reversals) which are not so fast as to be significant to laser operation. Representative prior disclosures of such systems are the following U.S. Patents: U.S. Pat. No. 5,467,363 to Okabayashi, U.S. Pat. No. 6,014,161 to Hirst et al., U.S. Pat. No. 6,043,835 to AuYeung, and U.S. Pat. No. 6,069,645 to Vincent.
For certain operations, particularly high-resolution laser printers, known prior calibrations are unsatisfactory. A high-quality printer operating at resolution of 1200 dots per inch data might operate the laser at four times that dots resolution, or 4800 time slices per inch (each operation or non-operation being termed a slice). Representative printers operating in slices are disclosed in U.S. Pat. No. 5,631,746 to Overall et al and U.S. Pat. No. 5,729,270 to Wright et al.
Variation from the desired level of optical output has a direct impact on print quality and also print consistency from machine to machine. Variation in any of the system components such as the source of the laser on/off (duty cycle) signal, any passive components on the on/off signal, cabling (if any) and the laser diode itself can have a significant influence on the actual laser light energy produced, especially if the laser on/off signal is modulated quickly enough that edges occur within 10 ns of each other. The laser energy output could be more tightly controlled in an open-loop manner in a variety of ways such as tightening component tolerances, slowing down the modulation rate of the on/off signal, eliminating the cable required to take the on/off signal from its source to the laser diode driver or by using fiber-optic or differential drivers on the on/off signal. However, laser printers are generally cost-sensitive and these other solutions can be costly and only serve to minimize variation by controlling the input to the laser diode driver rather than by sampling the actual energy output level and adjusting the system accordingly.
In accordance with this invention calibration extends to the differences experienced at different duty cycles and through a range of currents driving the laser at those duty cycles. Moreover, this invention employs a realization that at lower speeds, variations from the PIN diode between a batch of lasers manufactured to be the same, are variations of the PIN diode system and not of the optical output. The output of such lasers is substantially identical, and the calibration of this invention adjusts for that.
Printers are necessarily designed assuming a given laser operation. This is termed the nominal laser operation, and specified nominal values are the average or central values of those specified by the manufacturer. In accordance with this invention the values of a nominal laser are determined by observation of a representative batch of lasers at operations between a low current (such as 25 percent of maximum specified) and a high value (such as 85 percent of maximum of specified). This is observed at the lowest duty cycle employed by the printer (such as one 4800 slice per inch), and at the higher duty cycles (such as two adjoining of the 4800 slices, which is a duty cycle of power-on for {fraction (1/2400)} inch; also three adjoining of the 4800 slices; and finally four adjoining of the 4800 slices). Additionally, the PIN diode value of the nominal printer is measured while driving the printer at 100% duty cycle at medium power.
At manufacture of a printer, these same values are measured for just the actual laser of that printer. During printing the output is adjusted by determining the PIN value of the nominal printer for each slice to be printed, adjusting that value to reflect the PIN value of the laser of the printer, and then determining the slice and current in the printer to produce that adjusted PIN value from the laser. (As a printer may only print by regular slices, the slices may be more or less than the data for the nominal printer specifies, with the drive current being proportionally lower or higher.)